1. Field of the Invention
The present invention relates to piezoelectric resonators, piezoelectric filters, and duplexers and communication apparatuses including the same. More specifically, the present invention relates to a piezoelectric resonator and piezoelectric filter that are used in filters, oscillators, and communication devices and that generate thickness longitudinal vibration to function as filters in the VHF band, the UHF band, or at ultrahigh frequencies above such bands, and to a duplexer and a communication device including the same.
2. Description of the Related Art
Recently, filters that are used for high-frequency stages (RF, particularly frequencies above the GHz band) of communication devices such as cellular phones have been developed using piezoelectric resonators having favorable characteristics. The favorable characteristics include small size and light weight, vibration endurance and shock endurance, precision and reliability of products, automatic and simplified mounting allowed by the elimination of the need for adjusting circuitry, and ease of manufacturing high-frequency components.
For example, a piezoelectric resonator includes a substrate having an opening or a concavity, a diaphragm implemented by an insulating thin film disposed over the opening or concavity of the substrate, and a vibrating section in which at least one pair of an upper electrode and a lower electrode oppose each other so as to sandwich an upper surface and a lower surface of a thin-film section having at least one layer of a piezoelectric thin film. This type of piezoelectric resonator is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 2001-168674, published on Jun. 22, 2001. In the piezoelectric resonator, because the piezoelectric resonator has a piezoelectric member implemented by thin films, the high frequency operating range can be increased to several hundred MHz to several thousand MHz.
In the piezoelectric resonator described above, the dimensions of electrodes must be restricted within a predetermined range in order to suppress unwanted vibration. Furthermore, in order to trap vibration energy in the vibrating section, the dimensions of the electrodes must be small relative to the dimension of the diaphragm. Thus, when a high voltage is applied, the voltage is concentrated in the small vibrating section, generating a relatively large amount of heat in the vibrating section.
However, according to the related art, since the diaphragm constituting the vibrating section is formed by thin films and has a small heat capacity, the heat that is generated is hardly dissipated, and most of the heat is accumulated in the vibrating section, causing an increase in the temperature of the vibrating section. The temperature increase in the vibrating section could possibly cause destruction of the vibrating section, degrading the stability of operation.